There is a scarcity of versatile detectors which are efficient for detecting .alpha., .beta., and .gamma. radiation. Such a detector should have applications as a research-quality detector, as well as applications for use in routine applications, such as monitoring nuclear material, monitoring polluted areas, and nuclear medicine with relatively unskilled workers (in comparison to nuclear and particle-physics experimentalist). The class of detectors called counting tubes (ionization chambers, proportional counters and Geiger-Mueller detectors) are commercially available for detection of .alpha., .beta., and .gamma., radiation. However, counting tube instruments are subject to a number of disadvantages. They are fragile for use in the field under harsh and unstable conditions. They are inefficient for detecting .gamma. rays (actually X-rays). Detection efficiency for .gamma. rays above 30 keV drops dramatically for proportional counters. Where there are both alpha and beta radiation present together, there is little chance of finding the amount of beta radiation without extra equipment, since alpha radiation dominates the detector response. The need for a versatile detector arises from requirements including physical limitations, cost effectiveness and simplicity.
Another class of detectors in the prior art employ a combination of scintillators, in series, such that the scintillation light from each scintillator is detectable by a single photomultiplier tube. Because their configuration is essentially a `sandwich of phosphors`, such detectors are often referred to as `phoswich` detectors. Differentiation phoswich detectors operate on the principle that the response of one scintillator is separable from the response of the other scintillator, and also separable from the response of the combination. The method of achieving this separation has been to optically couple a scintillator with slow response to a scintillator with fast response, such that their responses can be separated.
Plastic scintillators have low atomic number, and are efficient detectors of .beta. radiation. In this regard, plastic scintillators are more efficient than high-Z materials, because the latter tend to backscatter electrons. Plastic scintillators also have a major advantage in that they are flexible, and are used in varied applications. Their low density and low atomic number, however, render them less effective with .gamma. radiation.
Inorganic crystal scintillators have medium to high stopping power due to their high density and atomic numbers. This, and other attributes, make them suitable for detecting .gamma. rays. Examples include BaF2, CeF3, CsF2, CsI(TI), CsI(Na), Bi4Be3012 (BGO), ZnS(Ag), and NaI(TI). NaI(TI), CsI(TI) are alkali-halide crystals doped with a small molecular concentration of thallium (.about.10-3 TI). The thallium doping induces the CsI(TI) crystal to emit light in the wavelength region around 500 nanometers, and that light has a decay constant of between 0.3 to 1.0 microseconds. CsI(pure) is growing in popularity for research, because of its excellent response time, its ability to be grown into large quantities, and cost, among other attributes.
A combination of scintillators, provides properties of versatile particle detection which have been utilized in other inventions. Goldsworthy (U.S. Pat. No. 2,727,154) utilizes two crystals in combination. Constraints on its use include the tasks of changing voltage, and changing the window through which radiation is incident. Christianson (U.S. Pat. No. 3,299,267) utilizes two luminophores, essentially two plastic scintillators in combination, with the limitation that .gamma. rays are detected efficiently only if they are below a maximum energy of about 20 keV. Ciles et al. (U.S. Statutory Invention Registration H590) utilizes a thin glass in combination with a thick plastic, The result of this combination is a tradeoff between the bulk of the detector with the efficiency of .gamma.-ray detection.
In most prior art phoswich detectors, the fast response scintillator is placed ahead of the slow one, i.e., the fast response one is the first to be struck by the radiation. In a more recent development, I proposed the use of a CsI crystal as the fast phosphor, and a reverse of the arrangement so that the slow plastic scintillator was positioned first and the fast CsI crystal second. This arrangement was proposed to take advantage of the high speed properties of CsI, but that type of detector proved to have certain disadvantages involving longer decay, due to the signal decay time of perhaps 300 nanoseconds for the plastic scintillator, and poor timing resolution due to the slow rise time of the plastic scintillator pulse.